It is highly unlikely that the lilac will cause damage to a foundation. They have very non-aggressive root systems and this is why you often see them used as foundation plants. The only way that it could be a threat (and even then, the plant would probably die before it did damage) would be if the roots were restricted tightly, as in there were only a few feet for the roots to grow in. That does not sound like the case where you are, so the lilac is not a threat.